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FT-5000 with AIRSPY HF+ #spectrum

valerio spagnolo
 
Edited

Hi, i'm using new spectrum scope feature through IF output (9 Mhz) of FT-5000.
I have connected an Airspy HF+ and set ok, but doesn't work properly.
RX frequency signal displayed in the spectrum is not alligned perfectly with the CAT freq of radio, sometimes i have to change IF from 9000 khz to 8999.7 / 9000.2.

Is it better to use RX out than IF of FT-5000 ?

Anyone have this problem like me?

73' de IU3BTY Val

ve3ki
 

Any time you use the IF as the source for an SDR spectrum display there will be a mode-dependent offset. In AM or CW, the offset is zero, because the radio's dial frequency corresponds exactly with the centre of the IF bandpass. In USB, however, the radio's dial frequency is approximately 1500 Hz below the frequency that is in the centre of the bandpass. In LSB, the radio's dial frequency is approximately 1500 Hz above the frequency in the centre of the bandpass. In order to label the frequencies in the display correctly, the software has to apply an offset depending on the mode. The sign of the offset depends on the sideband, and also on whether the radio's local oscillator frequency is above or below the actual radio frequency..

In other words, in order to label the frequencies in the spectrum display accurately, in SSB modes the software has to add an offset of approximately 1500 Hz to the displayed frequencies in LSB, and subtract a similar offset from the displayed frequencies in USB. Another way to do this is for the software to tune the SDR to a frequency that is different from the actual IF by an amount equal to the size of the offset - this will have the same effect in the end.

The precise value of the offset depends on your IF bandwidth settings. If your IF bandwidth is set to 2.8 kHz, your IF bandpass is between approximately 200 Hz and 3000 Hz. With this bandpass, the centre frequency is 1600 Hz above or below the dial frequency, so the software needs to use an offset of +1600 Hz in LSB and -1600 Hz in USB. If instead your IF bandwidth is set to 2.1 kHz, your IF bandpass will be between approximately 200 Hz and 2300 Hz, so the software will need to use an offset of +1250 Hz in LSB and -1250 Hz in USB.

In other words, in SSB modes the correct offset varies as you adjust the IF bandwidth. If the software uses a fixed per-mode offset when it calculates the frequencies to display below the spectrum, those frequencies will only be accurate for one particular IF bandwidth setting. At other bandwidth settings, there will be an error of up to a few hundred Hz. This could be the source of the error you reported.

This variable offset only happens when you use IF as the source for the SDR. When you use RF (RX Ant Out), the SDR is simply tuned to the same frequency as the radio's dial regardless of the mode, and there is no need to apply an offset when calculating the frequency to display below the spectrum. There are other advantages and disadvantages to each method (RF vs IF feed), but a significant disadvantage to using IF is the need to use a variable offset depending on the mode and filter settings.

73,
Rich VE3KI


On Mon, Oct 21, 2019 at 05:59 AM, <iu3bty@...> wrote:
Hi, i'm using new spectrum scope feature through IF output (9 Mhz) of FT-5000.
I have connected an Airspy HF+ and set ok, but doesn't work properly.
RX frequency signal displayed in the spectrum is not alligned perfectly with the CAT freq of radio, sometimes i have to change IF from 9000 khz to 8999.7 / 9000.2.

Is it better to use RX out than IF of FT-5000 ?

Anyone have this problem like me?

73' de IU3BTY Val

John, G4DRS
 

Rich,
That's the clearest explanation of offsets I have ever read.

Thanks.


John
G4DRS

valerio spagnolo
 

Thanks Rich your kind answer, really complete!
Now I understood the reason.

Ciao and agn tnx, IU3BTY